1. Field of the Invention
This invention relates to a heat stable fiber forming polyester and a new and novel process for preparing it. More particularly, this invention relates to an improved linear high molecular weight heat stable polyester especially suitable for preparing fibers which have excellent resistance to degradation when utilized in commercial articles, such as tires, industrial belting, etc., wherein a high degree of heat is built up during use.
2. Description of the Prior Art
High molecular weight polyethylene terephthalate fiber forming polyesters are well known. They are prepared commercially either by the ester interchange reaction between dimethyl terephthalate and ethylene glycol or by the direct esterification process wherein terephthalic acid is reacted directly with ethylene glycol. These products and processes are well documented in U.S. Pat. Nos. such as 2,465,310; 3,050,533; 3,051,212, 3,427,287, and 3,484,410 which cover not only the basic products and processes but many improvements thereon.
Polyethylene terephthalate fibers and cords are known to exhibit excellent dimensional stability, that is, low extension or growth during service, as well as to have a high resistance to thermal degradation; however, in pneumatic tires and industrial belts under high speed conditions under heavy load, loss of tensile strength is experienced due to high temperature conditions emanating under such conditions. Efforts to remedy this problem have all too often been ineffective. Most research in this field has been directed to producing a high molecular weight linear polyester having a low content of free carboxyl groups. The following patents are pertinent.
U.S. Pat. 3,051,212 to William W. Daniels relates to reinforced rubber articles and to textile cords and fibers for reinforcing such articles. This patent discloses that a linear terephthalate polyester having a concentration of free carboxyl groups of less than 15 equivalents per million grams may be prepared in a number of different ways. One effective procedure is to treat the filaments, after they have been formed, with a chemical reagent which reacts with and "caps" the free carboxyl group. One such agent is diazomethane.
U.S. Pat. No. 3,627,867 to Eckhard C. A. Schwarz discloses a process and apparatus for melt spinning high molecular weight polyethylene terephthalate into high performance fibers under conditions which reduce the normally high viscosity of such polyester. Ethylene oxide or other lowboiling oxirane compound is injected under pressure into molten polyester before it is fed to the metering pump of the melt spinning machine. The fibers are characterized by low free-carboxyl content and freedom from voids which might be expected from injection of the volatile material.
U.S. Pat. No. 3,657,191 to Rudolph Titzmann et al. is directed to a process for the manufacture of linear polyesters having an improved stability with respect to compounds with active hydrogen. Polyesters of this type are obtained by reacting polyesters with ethylene carbonates or monofunctional glycidyl ethers. The reaction is first carried out within a temperature range lying 10.degree. C. to 60.degree. C. below the softening point of the polyester and is then terminated during the melting and melt-spinning process. U.S. Pat. No. 3,869,427 to Robert W. Meschke et al. discloses a process of preparing polyester filaments having low free-carboxyl group contents which give superior performance in pneumatic tires and other reinforced rubber articles where heat degradation is a problem. Reduction of free carboxyl groups is achieved by mixing with the molten polyester, prior to melt-spinning, 1,2-epoxy-3-phenoxypropane or 1,2-epoxy-3-n-hexyloxypropane.
U.S. Pat. No. 4,016,142 to William Alexander et al. discloses preparation of a fiber-forming polyester wherein the number of free carboxyl end groups present in the polymer may be reduced by adding to the polymerized polyester a glycidyl ether which reacts with the carboxyl end groups present to form free hydroxyl end groups.
The subject of U.S. Pat. No. 4,139,521 high molecular weight polyester using a carboxyl scavenger, N-(2,3-epoxypropyl)phthalimide, has many desirable characteristics. However, the scavenger has not been used in a commercial process because it has been found to be a mutagen in the Ames test. Since it is known that small changes in structure can reduce or eliminate mutagenicity (D. R. Wade, S. C. Airy, J. E. Sinsheimer, Mutation Research 58, pp. 217-223 [1978]), a program was initiated to modify N-(2,3-epoxypropyl)-phthalimide in order to reduce its mutagenicity. Modifications were also carried out with phenyl glycidyl ether, a commercially used polyester carboxyl scavenger which is not only positive in the Ames test, but which is an experimental animal carcinogen.
N-(2,3-epoxypropyl)phthalimide, phenyl glycidyl ether and seven other compounds which were synthesized in our laboratories are listed in the summary table along with results of laboratory evaluation tests on their carboxyl scavenging abilities. It is shown that four of the modified compounds retain their ability to react efficiently with polyester carboxyl end groups. Synthetic schemes used to prepare the modified epoxides in the laboratory are shown as follows. ##STR1##